Process for preparing unsaturated nitriles from alkanes

ABSTRACT

Unsaturated nitriles, particularly acrylonitrile or methacrylonitrile, are produced in a process which integrates dehydrogenation of an alkane, particularly propane or isobutane, to the corresponding olefin, followed by ammoxidation of the olefin in the dehydrogenation reactor effluent to the corresponding nitrile. After recovery of the nitrile product, the residual gases are processed to remove hydrogen, oxygen, and carbon oxides, after which the gases are recycled to the dehydrogenation reactor. By operating with relatively low conversion of olefin to nitrile in each pass, the overall efficiency of the process is improved despite the need to recirculate substantial amounts of unreacted hydrocarbons.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application 701,725, filed Feb. 14,1985, now U.S. Pat. No. 4,609,502, issued 9-2-86.

PRIOR ART

This invention relates to the preparation of unsaturated nitriles fromthe corresponding alkanes, particularly acrylonitriles from propane andmethacrylonitrile from isobutane.

While commercial plants ammoxidize propylene to acrylonitrile, manypatents have been obtained on processes which employ propane as a feed.Representative of these are U.S. Pat. Nos. 3,365,482, 4,010,188 and4,036,870. Such one-step processes are of less commercial interest sincethe yield of acrylonitrile from propane is lower than is usual withpropylene, thus offsetting the advantages of the lower cost feedmaterial.

A feed mixture of saturated and unsaturated C₄ compounds was used inU.S. Pat. No. 3,998,867, producing both methacrylonitrile and1,3-butadiene simultaneously. Including n-butane is said to increase theyield of methacrylonitrile from isobutylene, while dehydrogenating then-butene to butadiene. The effect of butanes in the feed was notreported.

In U.S. Pat. No. 3,433,823 alkanes are used as feedstocks for nitriles.Two separate catalysts are employed, one of which must be a vanadiumphosphate. The patentees apparently intended to carry out oxidation andammoxidation simultaneously.

The composition of the feed to an ammoxidation process may have animportant effect, as indicated in U.S. Pat. No. 3,535,366, where addinga heat carrier gas, such as methane, ethane, and carbon dioxide, isshown to moderate the "hot spot" in the fixed bed ammoxidation ofxylenes to terephthalonitrile.

Improved yield of nitriles was obtained by adding carbon monoxide to thereactants, according to U.S. Pat. No. 3,868,400.

An integrated two-step process for production of unsaturated nitrilesfrom alkanes is suggested in U.S. Pat. No. 3,161,670, where it is shownthat the presence of hydrogen formed by dehydrogenating propane topropylene was not detrimental to the subsequent ammoxidation of thepropylene to acrylonitrile. The effect of unreacted propane and steam isnot considered, nor is a recycle process discussed.

Disadvantages associated with processes which convert alkanes tonitriles have limited their use, even though alkanes are generally lessexpensive feedstocks than the corresponding olefins. An efficientprocess for producing nitriles from alkanes has been found by thepresent inventors, as will be seen from the following description.

SUMMARY OF THE INVENTION

In an improved process for preparing nitriles, the corresponding alkaneis dehydrogenated to the olefin over a Group VIII noble metal catalystin the presence of steam, after which oxygen and ammonia are added tothe effluent of the dehydrogenation step and the mixture passed over anammoxidation catalyst to produce the nitrile. The product is absorbedfrom the ammoxidation effluent by an aqueous stream and recovered. Thenitrile-depleted effluent is processed to selectively oxidize thehydrogen produced by dehydrogenating the alkane and to separate the netproduction of carbon oxides, after which the depleted effluent streamcontaining unreacted alkane and olefin is returned to thedehydrogenation reactor.

The process is of particular interest in the preparation ofacrylonitrile from propane via propylene or alternatively preparation ofmethacrylonitrile from isobutane. When propane is the feedstock thedehydrogenation step is fed with propane and steam in a mol ratio ofabout 1/0.1 to 1/10 and at a temperature of about 400°-700° C. and apressure about 0.1-5 bar. An average of about 20-60% of the propane isconverted per pass, with a selectivity to propylene of about 92-98%.

The ammoxidation step is operated at an outlet temperature of about375°-550° C. and a pressure of about 0.1-10 bar. The conversion ofpropylene to acrylonitrile is about 30-80% for each pass, preferablyabout 40-75%, and the selectivity to acrylonitrile is about 80-90%.These conditions contrast with the conventional once-through processes,which convert about 93% of the propylene with a selectivity of about 73%to acrylonitrile. Similar conditions are used when isobutane isdehydrogenated to isobutylene and then ammoxidized to methacrylonitrile.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in a block diagram a process of the prior art.

FIG. 2 illustrates in a block diagram the process of the invention.

FIG. 3 provides a flowsheet showing one embodiment of the processillustrated in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the process of the invention, an alkane, especially propane orisobutane, is dehydrogenated to the corresponding olefin and thereafter,oxygen and ammonia are added to the effluent of the dehydrogenationreaction and the mixture passed over a suitable catalyst to prepare thecorresponding nitrile.

The block diagram of FIG. 1 illustrates the simplicity of the prior artprocess for making acrylonitrile from propylene. Propylene, air, andammonia are fed to a catalytic reactor where a high yield ofacrylonitrile is obtained. The product is recovered and purified byconventional absorption and distillation procedures. Since the yield ofacrylonitrile is very high, any unconverted propylene may be disposedof. The analogous process for making methacrylonitrile from isobutanemay also be carried out in a similar manner.

When one skilled in the art reviews the teaching of experts in theproduction of acrylonitrile, it becomes clear that, since the process issimple and the conversion is high, no incentive is seen forimprovement--except possibly in the development of even more efficientcatalysts. Generally, the catalysts used, such as antimony-uraniumoxides, provide about 93% conversion of the propylene fed. Consequently,recycling to improve conversion of the propylene and selectivity toacrylonitrile is unattractive. One skilled in the art would concludethat recycling unreacted propylene would be technically feasible, butnot cost effective since the raw material saved does not pay for costsof recycling.

If a skilled worker considered using propane as a feedstock, he wouldconclude that it is not cheap enough to justify the expense ofdehydrogenating it to propylene, which is available at low cost as oneof the products of steam-cracking naphtha or from natural gas liquids.If propane is used as a feedstock directly, as has been suggested insome patents, the yield of acrylonitrile is too low to provide aneconomical process. Since the commercial process is operated with highefficiency using propylene as a feedstock, propane would be seen as aless attractive choice.

The present inventors have found that the conventional wisdom outlinedabove leads to the wrong conclusions, and that propane can beefficiently converted to acrylonitrile. If propane is dehydrogenated topropylene and then fed along with ammonia to an acrylonitrile reactor, asubstantial amount of propane remains. For efficient operation, arecycle of unconverted propane, which will be associated with anyunconverted propylene, can be established to provide essentiallycomplete conversion of the propane to acrylonitrile and byproducts.Then, if the conversion of propylene to acrylonitrile is kept unusuallylow, say, between 30 and 80%, so that the selectivity to acrylonitrileis higher than the usual 73%, for example, about 80-90%, the result isan efficient process, which, despite its complexity, is capable oflow-cost production of acrylonitrile.

Methacrylonitrile may be produced from isobutane in an analogousprocess. FIG. 2 generally illustrates such a process as applied to apropane feedstock. Propane and steam are fed to a dehydrogenationreactor where about 20-60% of the propane is converted to propylene,along with some byproducts, such as hydrogen, carbon oxides, methane,ethane, and ethylene. A substantial amount of propane remainsunconverted, making it economical to recover and recycle it. Thedehydrogenation takes place at conditions known in the art, that is,about 400°-700° C. and about 0.1-5 bar, over a supported Group VIIInoble metal catalyst, usually including promoters. The reaction is quiteendothermic, and more than one reactor may be used with reheatingprovided between them.

The dehydrogenation reactor effluent is fed directly to the ammoxidationreactor, along with added oxygen and ammonia. The steam content may beadjusted as desired. Again, the reaction is carried out at conditionsknown in the art, that is, temperatures of 375° to 550° C., pressures ofabout 0.1-10 bar, with ammonia to propylene mol ratios of 0.2/1 to 2/1.The catalyst may be any of those known in the art, but antimony-uraniumcompositions are preferred, employed either in a fixed-bed tubularreactor or in a fluidized bed reactor. It is characteristic of theprocess of the invention that, instead of obtaining the maximum yield ofacrylonitrile in each pass through the ammoxidation reactor, theconversion is lowered from the maximum possible into the range of about30 to 80%, preferably about 40-75%, while the selectivity toacrylonitrile becomes about 80 to 90%, depending upon the catalyst, itscondition, and the operating conditions chosen. While operating underthese unique conditions does increase the recycle of unreacted propaneand propylene, it produces a larger net yeild of acrylonitrile for eachmol of propane fed. This method has been found to provide more efficientproduction of acylonitrile than the simple once-through process of FIG.1, despite being contrary to conclusions expected by those skilled inthe art, as previously discussed.

Acrylonitrile is recovered and purified from the reactor effluent gasesby conventional means. The residual gases include the unreacted propaneand propylene, plus hydrogen, oxygen, carbon oxides, water, byproductssuch as acetonitrile and hydrogen cyanide, and light hydrocarbons. Thenet hydrogen produced in dehydrogenating propane must be purged, alongwith carbon oxides. The residual oxygen cannot be returned to thedehydrogenation reactor, and it is selectively reacted with the hydrogenin a separate reactor over a catalyst capable of reacting hydrogen andoxygen without burning the propane and propylene. Such catalystspreferably are supported noble metals, such as platinum. The oxidationof hydrogen is carried out at suitable temperatures up to about 40°-550°C.

After the residual oxygen has been consumed by oxidizing hydrogen,sufficient gases will be purged to remove the net production of carbonoxides and light hydrocarbons. The purge gas is treated to recover thepropane and propylene which it contains and is then disposed of. Afterremoving the purge gases, the remainder of the effluent gases arerecycled to the dehydrogenation reactor.

Where the process is applied to an isobutane feedstock, the sameprinciples apply although the operating conditions will differ somewhat,as will be appreciated by those skilled in the art.

A simplified flowsheet is shown in FIG. 3 which will provide an exampleof a practical embodiment of the invention as it is a propane feedstock.Similar conditions would apply is isobutane were the feedstock. Freshpropane feed 10 is added upstream of selective oxidation reactor 58 toabsorb some of the heat of reaction, and combined with recycle stream46. The effluent of reactor 58 is sent to the dehydrogenation reactor 18after a purge stream (64) is removed. The hydrocarbons recovered frompurge stream 64 are returned via stream 80. The stream needed issupplied by the recycle gases, although additional steam (12) may beadded if needed. The combined stream 14 is then heated in exchanger (orfurnace) 16 to a temperature suited for the dehydrogenation of propaneto propylene. The feedstream contains propane and steam in molar ratiosbetween 1/0.1 and 1/10, preferably 1/0.2 to 1/4, and is fed attemperatures between about 400°-700° C. preferably about 600° C., and atpressures about 0.1-5 bar, to reactor 18 where about 20-60% of thepropane is converted to propylene with a selectively of 92-98%,depending upon the conditions chosen. The dehydrogenation reaction isendothermic, and the temperature leaving the reactor 18 will be lowerthan the inlet temperature. Multiple beds of catalyst with interstageheating of the gases may be used.

A number of catalysts have been disclosed in the prior art for use inthis process, and the conditions under which the reaction is carried outwill vary with the catalyst selected. Particularly useful is aplatinum-based catalyst of the type shown in U.S. Pat. No. 4,005,985.Although platinum and tin disposed on a zinc aluminate support providegood performance, other catalysts which have been found effectiveinclude platinum and rhenium or indium supported on zinc aluminate.Other Group VIII noble metals, alone or in combination on varioussupports known to the art, may have application in the dehydrogenationof propane to propylene. Other potential supports would include alumina,other alkaline earth metal aluminates, and rare earth aluminatesincluding lanthanum. Promoters such as tin, lead, antimony, and thalliummay be used. Base metal catalysts, such as the chromium, zirconium,titanium, magnesium, and vanadium oxides shown in U.S. Pat. Nos.3,479,416 and 3,784,483, or the zinc titanate of 4,176,140 and4,144,277, also might be used. The invention is not considered to belimited to specific catalyst formulations.

It will be understood by those skilled in the art that this processinvolves a rapid deactivation of the catalyst, and typically the processwill be operated with multiple reactors so that frequent regeneration ispossible. The details of such operations are, however, not consideredpart of the invention.

The dehydrogenation reactor effluent is cooled by generating steam (20)to a suitable temperature for inlet to the ammoxidation reactor 22 andjoined with oxygen (25) and ammonia (27) to provide a suitable feed forthe ammoxidation of propylene to acrylonitrile. Substantially pureoxygen is preferred, although less pure oxygen could be used with acompensating increase in the purge gas removed. If the dehydrogenationreaction is operated at low pressures, a compressor (21) may berequired. The reaction would be carried out under conditions typical ofthe art, that is, temperatures in the range of about 375°-550° C.,pressures of about 0.1-10 bar. A suitable ammoxidation catalyst will beused, typically a mixture of base metal oxides, especially those whichcomprise antimony and uranium and include promoter elements. The reactormay be of the tubular type where the pelleted catalyst is placed insidetubes which are surrounded by a heat transfer fluid for the removal ofthe heat of reaction. Preferably, a fluid-bed reactor may be used.Typically, 30-80% of the propylene feed to the reactor will be convertedto acrylonitrile, plus minor amounts of acetonitrile, hydrogen cyanideand light and heavier byproducts. A certain amount of the propylene isburned to carbon oxides and water.

The reactor effluent gases (24) may be cooled (26) by generating steamand fed to a quench tower 28 where any excess ammonia is reacted withsulfuric acid to form ammonium sulfate, which is scrubbed out at anaqueous stream. The aqueous ammonium sulfate (30) is steam-stripped incolumn 32 and removed as a bottoms product (35) including heavybyproducts. The overhead gases (36) which include some acrylonitrile,are sent of the acrylonitrile-acetonitrile splitter 34. Gases from thetop of quench tower 28 are compressed (38) and sent to an absorber tower40 where acrylonitrile is absorbed in a recirculating aqueous solutionintroduced as stream 42. The enriched solution passes as stream 44 tothe acrylonitrile acetonitrile splitter 34. After the nitriles have beenremoved, the residual gases pass overhead from the absorber tower 40 asstream 46 for subsequent removal of oxygen and hydrogen and purge ofcarbon oxides.

The acrylonitrile-acetonitrile splitter 34 separates crude acrylonitrileas an overhead product (48) contaminated with byproduct hydrogencyanide. The crude acrylonitrile is sent to a series of distillationcolumns for purification (not shown). The bottom product (53) isprimarily acetonitrile containing high-boiling by products and water.The acetonitrile is purified by distillation in column 52, being takenas an overhead product (54). The recirculating aqueous stream used toabsorb acrylonitrile is withdrawn from column 52 and sent to column 40as stream 42. Heavy byproducts are purged from the bottoms stream 56 andthe remainder is returned to the quench tower 28.

The effluent gas (stream 46) still contains significant quantities ofhydrogen made in the dehydrogenation of propane and excess oxygensupplied to the ammoxidation reactor. The oxygen is removed from theeffluent gas in oxidation reactor 58, which employs a catalyst capableof oxidizing hydrogen to water so that the C₃ components aresubstantially unaffected. Various oxidizing catalysts may be used forthis purpose, such as noble metal or base metals. In particular,platinum or palladium on alumina has been found particularly useful,since the reaction can be initiated at near ambient temperature.However, any convenient temperature up to about 550° C. might beemployed. Alternatively, platinum on a zeolite support sized to excludeC₃ hydrocarbons could be chosen. Such catalysts are capable ofcompletely oxidizing hydrogen to water without oxidizing C₃ components.Thus, the effluent stream is adjusted to the desired reactiontemperature by exchanger 60, fresh propane feed is added, and themixture is passed over the selective oxidation catalyst (58) for removalof oxygen and hydrogen. After purging the net production of carbonoxides and light hydrocarbons, the remaining gases are passed to thedehydrogenation reactor 18 to repeat the process, as previouslydiscussed.

Although oxygen has been removed, the gases still contain carbon oxidesmade during the ammoxidation reaction. Sufficient gas is purged viastream 64 to remove the net production of carbon oxides and lighthydrocarbons. The purge gas is first cooled in exchangers 66 and 68,condensing water which is removed in knockout drum 70. The remaining gasis compressed (72), cooled (74), and separated (76). Waste gases aredisposed of via stream 78, while water is purged from separator 76. Thepropane and propylene in the purge gas stream 64 are condensed inexchanger 74 and, being immiscible, are separated from the waterinseparator 76. They are passed (80) to vaporizer 82 before beingreintroduced into the recycle gas (62). Instead of the process justdescribed, the C₃ content of the gases might be recovered by absorptionin a suitable liquid. Also, carbon dioxide could be purged by absorbingit in an aqueous carbonate or other suitable solution.

An example of the practical operation of the flow-sheet shown in FIG. 3as applied to a propane feedstock is as follows:

One hundred mols/hr of a substantially pure propane feed stream (10) isvaporized and fed into stream 46 upstream of the selective oxidationreactor 58. Stream 46 totals 743 mols/hr and contains 14.9% hydrogen,7.1% oxygen, 0.7% methane, 9.2% ethane, 6.9% propylene, 23.9% propane,0.8% water, and 36.4% carbon oxides. The gas is sent at 60° C. to theselective oxidation reactor 58 where all of the oxygen is consumed. Thewater needed for dehydrogenation of the propane is supplied primarily bythe selective oxidation of hydrogen, although some additional steam maybe used. The effluent leaves at about 475° C., having been treated bycombustion of hydrogen. Thereafter, the residual gases (about 790.5mols/hr) are split and a purge stream of about 65.5 mols/hr is separatedand about 95% of the C₃ content recovered and returned to the recyclinggases (63). The carbon oxides, hydrogen, and other light gases arepurged (78). This represents the net production of these gases whichmust be removed to maintain the material balance and will vary asreactor conditions change. About 73.5 mols/hr of steam are added (12) tostream 62 to complete the feed to the dehydrogenation reactor (18),totalling 798.4 mols/hr and comprising 1.3% hydrogen, 31.3% carbonoxides, 0.6% methane, 8% ethane, 6.3% propylene, 34.6% propane, and17.6% water. The combined stream is fed to the dehydrogenation reactor18 at about 600° C. and 0.7 bar, and over a platinum and tin on zincaluminate catalyst about 35.7% of the propane fed is converted topropylene. Leaving the reactor at about 535° C., the effluent stream iscooled to about 150° C. in steam generator 20, compressed (21), andmixed with 214.5 mols/hr of oxygen (24) and 100.2 mols/hr ammonia beforebeing supplied to the ammoxidation reactor 22, where over a catalystabout 65.7% of the propylene is converted to acrylonitrile. Leaving thereactor 22 at about 405° C. and 2 bar, the effluent gases are cooled toabout 150° C. by generating steam and are passed to the quench tower 28where the residual ammonia is neutralized by sulfuric acid and somewater is removed. The remaining gases total about 1,208 mols/hr andcontain 9.2 % H₂, 4.3% O₂, 5.7% ethane, 4.2% propylene, 14.7% propane,22.4% carbon oxides, 6.7% acrylonitrile, 0.5% acetonitrile, 1% HCN, 31%H₂ O, plus minor amounts of various byproducts. This gas is compressed(38) and sent to the acrylonitrile absorber 40 where the productacrylonitrile is absorbed at about 40° C. and 6 bar in a recirculatingaqueous stream (42) of about 4,128 mols/hr.

The aqueous stream containing acrylonitrile (44) is distilled in column34 to produce a crude acrylonitrile stream (48) containing byproductHCN, which is purified by subsequent distillation columns (not shown).Byproduct acetonitrile is separated from the aqueous absorbing liquid bydistillation in column 52, with the aqueous stream being returned (42)to the absorber 40 for reuse.

Where isobutane is used as a feedstock, the operating conditions will besimilar, as will be appreciated by one skilled in the art.

What is claimed is:
 1. A process for the preparation ofmethacrylonitrile from isobutane comprising:(a) dehydrogenatingisobutane to isobutylene in the presence of 0.01 to 10 mols of steam foreach mol of isobutane, over a supported Group VIII noble metaldehydrogenation catalyst at a temperature of about 400°-700° C. and apressure of about 0.1-5 bar to form an effluent stream comprisingisobutylene, hydrogen, carbon oxides, steam, light hydrocarbons, andunreacted isobutane; (b) mixing oxygen and ammonia with said effluentstream of (a) and passing the mixture over an ammoxidation catalyst at atemperature of about 375°-550° C. and a pressure of about 0.1-10 barselected to convert about 30-80% of said isobutylene with a selectivityto methacrylonitrile of about 80-90%, and producing an effluent streamcomprising methacrylonitrile, unreacted isobutane and isobutylene,oxygen, hydrogen, steam, light hydrocarbons, and carbon oxides; (c)absorbing said methacrylonitrile from said effluent of (b) into anaqueous stream; (d) selectively oxidizing the hydrogen from themethacrylonitrile depleted effluent of (c) to water over a catalyst; (e)separating a portion of the effluent stream of (d) after said oxidationcontaining the net production of carbon oxides and light hydrocarbons,recovering the isobutane and isobutylene content thereof, purging thenet production of carbon oxides and light hydrocarbons and returningsaid recovered isobutane and isobutylene to the dehydrogenation step of(a); and (f) returning the effluent stream (d) after the separation of aportion thereof in (e) as feed to the dehydrogenation step of (a).